Exhaust pipe

ABSTRACT

An exhaust pipe includes a tubular portion and a space forming portion. The tubular portion forms an exhaust flow path in an exhaust system of an internal combustion engine. The tubular portion has a tubular shape. The space forming portion is provided along at least one of an inner surface and an outer surface of the tubular portion and forms at least one branch space between the space forming portion and the tubular portion. The at least one branch space communicates with the exhaust flow path and has a function as a side-branch muffler. The at least one branch space includes a space with a length that is one third or more of an entire length of the exhaust system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of Japanese PatentApplication No. 2020-208805 filed on Dec. 16, 2020 with the Japan PatentOffice, the entire disclosure of which is incorporated herein byreference.

BACKGROUND

The present disclosure relates to an exhaust pipe.

In exhaust systems of internal combustion engines, noise is reduced bymufflers provided in exhaust flow paths. For example, JapaneseUnexamined Utility Model Application Publication No. S61-138813discloses an exhaust system in which a branch pipe, which is dividedfrom an exhaust pipe forming part of an exhaust flow path, functions asa side-branch muffler.

SUMMARY

In such an exhaust system, low-frequency resonance that is dependent onthe entire length of the exhaust system may be generated as a type ofnoise. Unfortunately, in the exhaust system disclosed in S61-138813, thebranch pipe, which functions as a side-branch muffler, is short, therebyhindering noise reduction in the low frequency region.

It is desirable that one aspect of the present disclosure provides atechnique to facilitate noise reduction in the low frequency region.

One aspect of the present disclosure provides an exhaust pipe thatcomprises a tubular portion and a space forming portion. The tubularportion forms an exhaust flow path in an exhaust system of an internalcombustion engine. The tubular portion has a tubular shape. The spaceforming portion is provided along at least one of an inner surface andan outer surface of the tubular portion and forms at least one branchspace between the space forming portion and the tubular portion. The atleast one branch space communicates with the exhaust flow path and has afunction as a side-branch muffler. The at least one branch spaceincludes a space with a length that is one third or more of an entirelength of the exhaust system.

Such a configuration provides the branch space that functions as aside-branch muffler with a relatively long length, thereby reducingnoise in the low frequency region.

In one aspect of the present disclosure, the tubular portion may be alongest tubular portion of a tubular body forming the exhaust flow pathin the exhaust system.

In one aspect of the present disclosure, the at least one branch spacemay comprise a space with a length that is one third or more of theentire length of the exhaust system. Such a configuration provides aneven longer branch space that functions as a side-branch muffler,thereby further facilitating noise reduction in the low frequencyregion.

In one aspect of the present disclosure, the at least one branch spacemay comprise two branch spaces with lengths different from each other.Such a configuration has a muffling effect on sound waves with a widerrange of frequencies.

In one aspect of the present disclosure, the at least one branch spacemay comprise two branch spaces each having a length that is one fifth ormore of the entire length of the exhaust system. Such a configurationhas the muffling effect on sound waves with a wider range of frequenciesin the low frequency region.

In one aspect of the present disclosure, the space forming portion maybe provided along the inner surface of the tubular portion. Such aconfiguration allows the branch spaces to be formed with the outer shapeof the tubular portion maintained.

In one aspect of the present disclosure, the space forming portion maycomprise a tubular inner pipe member disposed inside the tubular portionand forming a double-wall pipe together with the tubular portion. Theinner pipe member comprises an attached portion and a separated portion.The attached portion is attached to the inner surface of the tubularportion. The separated portion is spaced apart from the inner surface ofthe tubular portion. The at least one branch space may comprise a spaceformed between the separated portion and the tubular portion. With sucha configuration, the branch space is formed by providing a double-wallpipe structure and, thus, can be relatively easily formed.

In one aspect of the present disclosure, an area defined by an outeredge of the separated portion in a cross-section perpendicular to anaxial direction of the double-wall pipe may be encompassed by an areadefined by an outer edge of the attached portion in the cross-section.Such a configuration allows the branch space to be formed with the outershape of the tubular portion maintained.

In one aspect of the present disclosure, the tubular portion maycomprise a curved portion. The curved portion forms a curve in theexhaust flow path.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the present disclosure will be describedhereinafter with reference to the accompanying drawings, in which:

FIG. 1 is a plan view showing an exhaust system of an internalcombustion engine;

FIG. 2A is an enlarged view showing a connection between a secondtubular member and a second catalytic converter;

FIG. 2B is an enlarged view showing a connection between a secondtubular member and a muffler;

FIG. 3A is a cross-sectional view taken along a line IIIA-IIIA in FIG.2A;

FIG. 3B is a cross-sectional view taken along a line IIIB-IIIB in FIG.2A;

FIG. 3C is a diagram showing the cross-section of the attached portionin FIG. 3A and the cross-section of the separated portion in FIG. 3B inan overlapping manner;

FIG. 4 is a schematic view showing the exhaust system of the internalcombustion engine of FIG. 1 in a simplified manner;

FIG. 5 is a graph showing a muffling effect of the exhaust system; and

FIG. 6 is a schematic diagram showing communication holes according to amodified example.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 1. Configuration

Referring to FIG. 1, an exhaust system 1 is disposed under the floor ofa vehicle. The exhaust system 1 forms an exhaust flow path that is aflow passage for exhaust gas discharged from an internal combustionengine 100 of the vehicle.

The exhaust system 1 comprises at least one exhaust part. As the atleast one exhaust part, the exhaust system 1 comprises, in the orderfrom the upstream side of the exhaust flow path (from the left side inFIG. 1), an exhaust manifold 2, a first catalytic converter 3A, a secondcatalytic converter 3B, and a muffler 4. The exhaust gas discharged fromthe internal combustion engine 100 flows through the exhaust manifold 2and into the first catalytic converter 3A. Each of the first catalyticconverter 3A and second catalytic converter 3B comprise a catalyst. Thefirst and second catalytic converters 3A, 3B reform or collectenvironmental pollutants in the exhaust gas. The muffler 4 reduces noisein the exhaust system 1.

The exhaust system 1 also comprises tubular members. Each tubular memberconstitutes a tubular body forming the exhaust flow path. Each tubularmember is connected to the at least one exhaust part at least at one endthe tubular member. Each tubular member forms the exhaust flow path inthe upstream or downstream of the at least one exhaust part of theexhaust system 1. As the tubular members, the exhaust system 1 comprisesa first tubular member 5A, a second tubular member 5B, and a thirdtubular member 5C. The first, second, and third tubular members 5A, 5B,5C are, for example, circular pipes each having a perfectly circularcross-section with an almost consistent outer diameter. The term“perfectly circular” as used herein does not mean to describe the shapeof a perfect circle in a strict sense, but means to distinguish acircular shape from an oval shape. The same applies hereinafter.

The first tubular member 5A is connected to the first and secondcatalytic converters 3A, 3B and forms the exhaust flow paththerebetween. The second tubular member 5B is connected to the secondcatalytic converter 3B and the muffler 4 and forms the exhaust flow paththerebetween. The third tubular member 5C is connected to the muffler 4and forms the exhaust flow path in the downstream of the muffler 4. Theexhaust gas that flows through the muffler 4 is discharged from thethird tubular member 5C to the outside of the exhaust system 1. In FIGS.1 to 2B, the direction in which the exhaust gas flows through theexhaust flow path is from left to right, that is, the directionindicated by an arrow Y.

At least one of the tubular members comprises a curved portion. Thecurved portion forms a curve in the exhaust flow path in the exhaustsystem 1. The exhaust system 1 is disposed under the floor of a vehicleas mentioned above. Under the floor of a vehicle, there are many othercomponents, such as a propeller shaft, cross members, side members, anda fuel tank of the vehicle, in addition to the exhaust system 1. Thus,in order to avoid interference with these components, the exhaust flowpath of the exhaust system 1 comprises the curved portion and, thereby,the exhaust flow path is at least partly curved as in exhaust systems ofcommon vehicles.

Specifically in the exhaust system 1, the first and second tubularmembers 5A, 5B are provided with the curved portion as shown in FIG. 1.Particularly, the second tubular member 5B is provided with curvedportions 51A to 51D at the positions where a double-wall pipe, whichwill be described below, is formed.

Out of the tubular members, the longest member is provided with a spaceforming portion, which will be described below. In the example shown inFIG. 1, the second tubular member 5B is the longest among the first,second, and third tubular members 5A, 5B, 5C. The second tubular member5B is, thus, provided with the space forming portion. Hereinafter, theconfiguration where the second tubular member 5B is provided with thespace forming portion is referred to also as an exhaust pipe 10 withmuffler function. The exhaust pipe 10 with muffler function correspondsto one example of the exhaust pipe. Moreover, the tubular memberprovided with the space forming portion, namely the second tubularmember 5B, corresponds to one example of the tubular portion.

The space forming portion is provided along at least one of the innersurface and the outer surface of the tubular portion, forming at leastone branch space. The at least one branch space refers to a space thatcommunicates with the exhaust flow path and functions as a side-branchmuffler.

As an example of the space forming portion that is formed along theinner surface of the tubular portion, the second tubular member 5B isprovided with a first inner pipe member 6A and a second inner pipemember 6B. Both the first and second inner pipe members 6A, 6B have atubular shape. Specifically, the first and second inner pipe members 6A,6B are circular pipes each having a perfectly circular cross-section.

Each of the first and second inner pipe members 6A, 6B is disposedinside the second tubular member 5B and forms the double-wall pipetogether with the second tubular member 5B. In other words, in theportion of the second tubular member 5B where the first inner pipemember 6A or the second inner pipe member 6B is disposed, thedouble-wall pipe is formed in which the second tubular member 5B servesas the outer pipe and the first inner pipe member 6A or the second innerpipe member 6B serves as the inner pipe. The first and second inner pipemembers 6A, 6B are not visible from the outside of the exhaust system 1,but shown with solid lines in the drawings to facilitate understanding.

As shown in FIGS. 2A, 3A, and 3B, the first inner pipe member 6Acomprises an attached portion 61A and a separated portion 62A. FIG. 3Ais a cross-sectional view showing a portion of the above-describeddouble-wall pipe composed of the second tubular member 5B and theattached portion 61A in which the portion is cut along a planeperpendicular to the axial direction of the double-wall pipe. Thecross-section in FIG. 3A (the cross-section taken along the lineIIIA-IIIA in FIG. 2A) is hereinafter referred to as a firstcross-section. FIG. 3B is a cross-sectional view showing a portion ofthe above-described double-wall pipe composed of the second tubularmember 5B and the separated portion 62A in which the portion is cutalong a plane perpendicular to the axial direction of the double-wallpipe. The cross-section in FIG. 3B (the cross-section taken along theline IIIB-IIIB in FIG. 2A) is hereinafter referred to as a secondcross-section.

As shown in FIGS. 2A and 3A, the outer surface of the attached portion61A is almost identical in shape to the inner surface of the secondtubular member 5B. The outer surface of the attached portion 61A is incontact with the inner surface of the second tubular member 5B along theentire circumference in the direction around the axis of the secondtubular member 5B. The outer surface of the attached portion 61A isattached to the inner surface of the second tubular member 5B bywelding. Both the first inner pipe member 6A and the second tubularmember 5B are, for example, circular pipes each having a perfectlycircular cross-section. Thus, the outer diameter of the attached portion61A is, for example, almost the same as the inner diameter of the secondtubular member 5B.

As shown in FIG. 3A, the area defined by an outer edge S1 of theattached portion 61A in the first cross-section nearly corresponds tothe area defined by an inner edge T of the second tubular member 5B inthe first cross-section. The expression of two areas nearlycorresponding to each other as used herein means that the two areas havealmost the same size, that is, the shapes and surface areas of the twoareas are almost the same.

As shown in FIGS. 2A and 3A, the outer surface of the separated portion62A is situated inside of the inner surface of the second tubular member5B in the exhaust flow path. The separated portion 62A and the secondtubular member 5B are spaced apart from each other. Both the first innerpipe member 6A and the second tubular member 5B are, for example,circular pipes each having a perfectly circular cross-section. Thus, theouter diameter of the separated portion 62A is smaller than, forexample, the inner diameter of the second tubular member 5B. The outerdiameter of the separated portion 62A is, for example, almost consistentexcept the portion of the separated portion 62A situated on the upstreamside of the exhaust flow path. The portion on the upstream side extendsto the attached portion 61A as will be described below, and graduallyexpands in diameter toward the attached portion 61A.

In the first inner pipe member 6A, the separated portion 62A is narrowerthan the attached portion 61A. The first inner pipe member 6A is, forexample, a circular pipe having a perfectly circular cross-section.Thus, the inner diameter of the separated portion 62B is smaller than,for example, the inner diameter of the attached portion 61A.

FIG. 3C is a diagram showing the cross-section of the attached portion61A in FIG. 3A and the cross-section of the separated portion 62A inFIG. 3B in an overlapping manner with the centers of these portionsaligned. As shown in FIG. 3C, the area defined by an outer edge S2 ofthe separated portion 62A in the second cross-section is encompassed bythe area defined by the outer edge S1 of the attached portion 61A in thefirst cross-section. A certain area is “encompassed” by another area asdescribed herein means that the size of the certain area is smaller thanthat of the other area. In other words, it means that the certain areacompletely fits into the other area when these areas are overlapped.

For example as shown in FIGS. 1 and 2A, the attached portion 61A formsthe upstream end of the double-wall pipe composed of the second tubularmember 5B and the first inner pipe member 6A. The separated portion 62Aforms the portion of the double-wall pipe situated downstream of theattached portion 61A. That is, a space 7A formed between the secondtubular member 5B and the separated portion 62A is closed in the axialdirection of the double-wall pipe at the upstream end of the double-wallpipe by the attached portion 61A. The space 7A formed between the secondtubular member 5B and the separated portion 62A communicates with theexhaust flow path at the downstream end of the double-wall pipe. Inother words, the space 7A formed between the second tubular member 5Band the separated portion 62A is included in the at least one branchspace described above. Hereinafter, the space 7A formed between thesecond tubular member 5B and the separated portion 62A is referred to asa first branch space 7A.

As shown in FIG. 2B, the second inner pipe member 6B comprises anattached portion 61B and a separated portion 62B. The attached portion61B and the separated portion 62B are configured similarly to theabove-described attached portion 61A and separated portion 62Arespectively. However, the portion of the separated portion 62B situatedon the downstream side of the exhaust flow path, instead of the portionon the upstream side of the exhaust flow path, extends to the attachedportion 61B, as will be described below. The outer diameter of theseparated portion 62B is almost consistent except the portion on thedownstream side. The portion on the downstream side gradually expands indiameter toward the attached portion 61B.

As shown in FIGS. 1 and 2B, the attached portion 61B forms thedownstream end of the double-wall pipe composed of the second tubularmember 5B and the second inner pipe member 6B. The separated portion 62Bforms the portion of the double-wall pipe situated upstream of theattached portion 61B. That is, a space 7B formed between the secondtubular member 5B and the separated portion 62B is closed in the axialdirection of the double-wall pipe at the downstream end of thedouble-wall pipe by the attached portion 61B. The space 7B formedbetween the second tubular member 5B and the separated portion 62Bcommunicates with the exhaust flow path at the upstream end of thedouble-wall pipe. In other words, the space 7B formed between the secondtubular member 5B and the separated portion 62B is also included in theat least one branch space described above. Hereinafter, the space 7Bformed between the second tubular member 5B and the separated portion62B is referred to as a second branch space 7B.

FIG. 4 is a schematic view showing the exhaust system 1 of FIG. 1 in asimplified manner. For easier understanding of the length of the branchspace, the exhaust system 1 in FIG. 4 is simplified in a manner suchthat both the first and second tubular members 5A, 5B have no curvedportions and the exhaust parts are linearly arranged. The length of eachbranch space refers to the length from the portion where the branchspace is closed to the portion where the branch space communicates withthe exhaust flow path.

In FIG. 4, the sound pressures of resonance that can be generated in theexhaust system 1 are also shown. Such resonance is a type of noise inthe exhaust system 1. The wavelength of resonance is dependent on alength La of the exhaust system 1. The term “length La of the exhaustsystem 1” as used herein means the entire length of the exhaust system1. In other words, the term “length La of the exhaust system 1” as usedherein means the entire length of the exhaust flow path in the exhaustsystem 1. In FIG. 4, the exhaust flow path in the exhaust system 1 isschematically shown in a linear manner. Thus, the length from an end Pa1to an end Pa2 corresponds to the length La of the exhaust system 1. Theend Pa1 is the end of the exhaust manifold 2 that is connected to theinternal combustion engine 100. The end Pa2 is the end of the thirdtubular member 5C that is open to the outside of the exhaust system 1.In the actual exhaust system 1, the exhaust flow path is not completelylinear as shown in FIG. 1. In such a case, the length along the shape ofthe exhaust flow path is the length of the exhaust system 1. Moreover,in the case where the exhaust flow path is, for example, curved insidethe exhaust part(s), the length of the exhaust system 1 is larger thanthe length of the appearance of the exhaust system 1 due to thelength(s) of the curve(s) and the like.

As the sound pressures of resonance that can be generated in the exhaustsystem 1, FIG. 4 shows specifically the sound pressures of a standingwave D1 in the first mode, a standing wave D2 in the second mode, and astanding wave D3 in the third mode. The standing wave D1 has an antinodeat the end Pa1, a node at the end Pa2, and no further nodestherebetween. One quarter of a wavelength λd1 of the standing wave D1 isequal to the length La of the exhaust system 1 ((¼)λd1=La). The standingwave D2 has an antinode at the end Pa1, a node at the end Pa2, and onemore node therebetween. One quarter of a wavelength λd2 of the standingwave D2 is equal to one third of the length La of the exhaust system 1((¼)λd2=(⅓)La). The standing wave D3 has an antinode at the end Pa1, anode at the end Pa2, and two more nodes therebetween. One quarter of awavelength λd3 of the standing wave D3 is equal to one fifth of thelength La of the exhaust system 1 ((¼)λd3=(⅕)La).

Side-branch mufflers having a side-branch space with a length Lbn thatis one quarter of the wavelength λ of noise (Lbn=(¼)λ) have the mufflingeffect particularly on such noise. Noise in the exhaust system 1 at alower frequency has a longer wavelength. In other words, noise in thelow frequency region has a longer wavelength. From the perspective ofreducing noise in the low frequency region, the exhaust system 1, thus,is provided with the branch space with a length that is one fifth ormore of the length La of the exhaust system 1. From the perspective offurther reducing noise in a lower frequency region, the exhaust system 1may be provided with the branch space with a length that is one third ormore of the length La of the exhaust system 1.

The first and second branch spaces 7A, 7B are designed, for example, tohave lengths different from each other. Specifically, a length Lb1 ofthe first branch space 7A is designed to be one fifth of the length Laof the exhaust system 1 (Lb1=(⅕)La). As described above, the firstbranch space 7A is closed on the upstream side of the exhaust flow pathby the attached portion 61A and communicates with the exhaust flow pathon the downstream side of the exhaust flow path. Thus, the length Lb1 ofthe first branch space 7A refers specifically to the length of theseparated portion 62A. In other words, the length Lb1 of the firstbranch space 7A refers to the length from an end Pb1 of the separatedportion 62A situated on the upstream side of the exhaust flow path to anend Pb2 of the separated portion 62A on the downstream side.

Moreover, a length Lb2 of the second branch space 7B is designed to beone third of the length La of the exhaust system 1 (Lb2=(⅓)La). Asdescribed above, the second branch space 7B is closed on the downstreamside of the exhaust flow path by the attached portion 61B andcommunicates with the exhaust flow path on the upstream side of theexhaust flow path. Thus, the length Lb2 of the second branch space 7Brefers specifically to the length of the separated portion 62B. In otherwords, the length Lb2 of the second branch space 7B refers to the lengthfrom an end Pb3 of the separated portion 62B situated on the upstreamside of the exhaust flow path to an end Pb4 of the separated portion 62Bon the downstream side.

2. Effects

According to the embodiment described in detail above, the followingeffects can be achieved.

(2a) In the exhaust system 1, the first and second branch spaces 7A, 7Bare formed. The first branch space 7A has a length that is one fifth ormore of the length La of the exhaust system 1. The second branch space7B also has a length that is one fifth or more of the length La of theexhaust system 1. Such a configuration provides the branch spaces, whichfunction as side-branch mufflers, with relatively long lengths, therebyreducing noise in the low frequency region. Such a configuration has themuffling effect particularly on the standing wave D3 in the third modeor sound waves with frequencies lower than that of the standing wave D3.

(2b) In the exhaust system 1, the second branch space 7B is formed. Thesecond branch space 7B has a length that is one third or more of thelength La of the exhaust system 1. Such a configuration provides thebranch space, which functions as a side-branch muffler, with an evenlonger length, thereby further reducing noise in the low frequencyregion. Such a configuration has the muffling effect particularly on thestanding wave D2 in the second mode or sound waves with frequencieslower than that of the standing wave D2.

(2c) In the exhaust system 1, the first and second branch spaces 7A, 7Bare formed. The first and second branch spaces 7A, 7B have lengthsdifferent from each other. Such a configuration has the muffling effecton sound waves with a wider range of frequencies.

(2d) In the exhaust system 1, the first and second branch spaces 7A, 7Bare formed. Each of the first and second branch spaces 7A, 7B has alength that is one fifth or more of the length La of the exhaust system1. Such a configuration has the muffling effect on sound waves with awider range of frequencies within a frequency region that includes thefrequency of the standing wave D3 in the third mode and lowerfrequencies.

Referring now to FIG. 5 which is a graph showing the muffling effect inthe exhaust system. In this graph, the horizontal axis represents thefrequency of a sound wave while the vertical axis represents the amountof noise reduction. The solid line shows the muffling effect of theexhaust system 1. The broken line shows the muffling effect of anexhaust system with no branch space. The three dash-dotted lines show,in order from low to high, the frequency of the standing wave D1 in thefirst mode, the frequency of the standing wave D2 in the second mode,and the frequency of the standing wave D3 in the third mode that can begenerated in the exhaust system 1. In the example shown in FIG. 5, thefrequency of the standing wave D1 in the first mode is about 42 Hz, thefrequency of the standing wave D2 in the second mode is about 125 Hz,and the frequency of the standing wave D3 in the third mode is about 208Hz.

As shown in FIG. 5, noise in the low frequency region is reduced more inthe exhaust system 1 than in the exhaust system with no branch space.The exhaust system 1 has a high muffling effect particularly on thestanding wave D3 in the third mode and the standing wave D2 in thesecond mode. The exhaust system 1 has the muffling effect also on asound wave with a frequency that is about three times higher than thatof the standing wave D2 in the second mode.

(2e) The first and second inner pipe members 6A, 6B are disposed alongthe inner surface of the second tubular member 5B. Such a configurationallows the branch spaces to be formed with the outer shape of the secondtubular member 5B maintained. It is, thus, possible to form the exhaustpipe 10 with muffler function that can be arranged in the same space asin the configuration where the second tubular member 5B merely functionsas an exhaust flow path. The configuration where the second tubularmember 5B merely functions as an exhaust flow path as mentioned hereinrefers to a configuration where the second tubular member 5B is notprovided with the first and second inner pipe members 6A, 6B and thefirst and second branch spaces 7A, 7B are not formed.

(2f) Each of the first and second inner pipe members 6A, 6B forms thedouble-wall pipe together with the second tubular member 5B. The firstbranch space 7A is formed between the second tubular member 5B and theseparated portion 62A of the first inner pipe member 6A. The secondbranch space 7B is formed between the second tubular member 5B and theseparated portion 62B of the second inner pipe member 6B.

With such a configuration, the branch spaces are formed by providing adouble-wall pipe structure and, thus, can be relatively easily formed.

(2g) The area defined by the outer edge S2 of the separated portion 62Ain the second cross-section is encompassed by the area defined by theouter edge S1 of the attached portion 61A in the first cross-section.Such a configuration allows the branch space to be formed with the outershape of the second tubular member 5B maintained. Thus, the externaldimension of the exhaust pipe 10 with muffler function is reduced ascompared with a configuration where a branch space is formed byexpanding the tubular portion in the circumferential direction of thedouble-wall pipe.

(2h) The second tubular member 5B is provided with curved portions 51Ato 51D at the positions where the double-wall pipe is formed, that is,where the first inner pipe member 6A or the second inner pipe member 6Bis disposed. Such a configuration allows the branch spaces to be formedirrespective of with or without the curved portions in the tubularportion, thereby providing longer branch spaces.

3. Other Embodiments

An embodiment of the present disclosure has been described hereinabove.The present disclosure, however, should not be limited to theabove-described embodiment and may be carried out in variously modifiedmanners.

(3a) In the above-described embodiment, two branch spaces, namely thefirst and second branch space 7A, 7B, are formed in the second tubularmember 5B. However, there may be one branch space, or three or morebranch spaces formed in the tubular portion.

(3b) In the above-described embodiment, the first and second branchspaces 7A, 7B are formed in the second tubular member 5B. The first andsecond branch spaces 7A, 7B have lengths different from each other. Inthe case where two or more branch spaces are formed in the tubularportion, each branch space may have the same length as the other branchspace(s) or may have a length different from the other branch space(s).That is, each of the branch spaces may reduce noise at the samefrequency as the frequency reduced in other branch spaces, or may reducenoise at a frequency different from the frequencies reduced in otherbranch spaces.

(3c) In the above-described embodiment, the first branch space 7Areduces noise at the frequency of the standing wave D3 in the thirdmode. The second branch space 7B reduces noise at the frequency of thestanding wave D2 in the second mode. In this manner, the aforementionedat least one branch space may comprise a branch space that reduces noiseat the frequency of at least one standing wave out of the standing wavesin the first to third modes.

(3d) In the above-described embodiment, the first and second branchspaces 7A, 7B have lengths that are one fifth or more of the length Laof the exhaust system 1. However, the lengths of the branch spacesformed in the tubular portion are not limited to one fifth or more ofthe length of the tubular portion. For example, the lengths of thebranch spaces formed in the tubular portion may be, for example, onethird or more, one half or more, two thirds or more, three quarters ormore, or four fifths or more of the length of the tubular portion.Moreover, the lengths of the branch spaces formed in the tubular portionmay be, for example, the same as the length of the tubular portion.

(3e) In the above-described embodiment, each space formed between thesecond tubular member 5B and the first inner pipe member 6A and betweenthe second tubular member 5B and the second inner pipe member 6B isclosed at one end of the double-wall pipe and communicates with theexhaust flow path at the other end, thereby forming the first and secondbranch spaces 7A, 7B. However, the branch spaces may be formed in adifferent way.

For example, the branch space may be formed as in an exhaust pipe 11with muffler function shown in FIG. 6. In the exhaust pipe 11 withmuffler function, the space formed between the second tubular member 5Band an inner pipe member 6C is closed at both ends of the double-wallpipe and communicates with the exhaust flow path through communicationholes 8 formed in the middle portion of the double-wall pipe. Thus, twobranch spaces 7C, 7D are formed.

In FIG. 6, a group of perfectly circular holes is shown as thecommunication holes 8. However, the number of holes constituting thecommunication holes is not particularly limited. The shape of the holesis likewise not particularly limited, and may be, for example, an ovalshape, a polygonal shape, a polygonal shape with round corners, or astar shape. The inner pipe member 6C is not visible from the outside ofthe exhaust pipe 11 with muffler function, but shown with solid lines inFIG. 6 to facilitate understanding.

(3f) In the above-described embodiment, the first inner pipe member 6Aor the second inner pipe member 6B forms the double-wall pipe togetherwith the second tubular member 5B. However, the tubular portion and thespace forming portion may form a multiple-wall pipe with, for example,triple walls or more.

(3g) In the above-described embodiment, the first inner pipe member 6Aor the second inner pipe member 6B forms the double-wall pipe togetherwith the second tubular member 5B. However, the space forming portiondoes not have to form a multiple-wall pipe, such as a double-wall pipe,together with the tubular portion.

In other words, each of the first inner pipe member 6A and the secondinner pipe member 6B is arranged inside the second tubular member 5B,covering the inner surface of the second tubular member 5B along theentire circumference in the direction around the axis of the secondtubular member 5B. However, the space forming portion may be providedinside the tubular portion, covering the inner surface of the tubularportion along part of the circumference in the direction around the axisof the tubular portion.

(3h) In the above-described embodiment, each of the first and secondinner pipe members 6A, 6B is provided along the inner surface of thesecond tubular member 5B. However, the space forming portion may beprovided along the outer surface of the tubular portion. In this case,the space forming portion may be provided outside the tubular portion,covering the outer surface of the tubular portion along the entirecircumference in the direction around the axis of the tubular portion.Alternatively, the space forming portion may be provided outside thetubular portion, covering the outer surface of the tubular portion alongpart of the circumference in the direction around the axis of thetubular portion.

(3i) In the above-described embodiment, out of the first, second, andthird tubular members 5A, 5B, 5C, the longest member, namely the secondtubular member 5B, is provided with the space forming portion. When thetubular member to be provided with the space forming portion isdetermined based on the length in this manner, the length of eachtubular member is acquired as follows. Specifically, if the tubularmember forms the exhaust flow path between the internal combustionengine and the exhaust part disposed at the uppermost stream position,the length of the tubular member is from the end of the tubular memberconnected to the internal combustion engine to the end connected to theexhaust part. If the tubular member forms the exhaust flow path betweentwo exhaust parts, the length of the tubular member is from the end ofthe tubular member connected to one of the exhaust parts to the endconnected to the other exhaust part. If the tubular member forms theexhaust flow path on the downstream side of the exhaust part disposed atthe lowermost stream position, the length of the tubular member is fromthe end of the tubular member connected to the exhaust part to the endopen to the outside of the exhaust system.

(3j) In the above-described embodiment, out of the first, second, andthird tubular members 5A, 5B, 5C, the longest member, namely the secondtubular member 5B, is provided with the space forming portion. However,the tubular member provided with the space forming portion is notlimited to the longest tubular member out of multiple tubular members.That is, the tubular portion does not have to be the longest portion ofthe tubular body forming the exhaust flow path in the exhaust system.

(3k) In the above-described embodiment, the second tubular member 5Bconsists of a single component. However, the tubular portion may becomposed of two or more components. For example, the tubular portion maybe a member comprising two or more tubular components coupled to eachother.

(3l) In the above-described embodiment, the first, second, and thirdtubular members 5A, 5B, 5C and the first and second inner pipe members6A, 6B have perfectly circular cross-sections. However, thecross-sectional shapes of the tubular members and an inner pipe memberare not particularly limited. The cross-sectional shapes of thesecomponents may be, for example, oval shapes, polygonal shapes, orpolygonal shapes with round corners. Moreover, these cross-sectionalshapes and the cross-sectional areas thereof may be variable.

(3m) In the above-described embodiment, the exhaust system 1 comprisesthe muffler 4; however, the muffler 4 may be optional.

(3n) In the above-described embodiment, the exhaust system 1 is used inthe internal combustion engine 100. However, the exhaust system 1 may beused in internal combustion engines of, for example, water vessels,aircrafts, and general-purpose machines.

(3o) Functions of one component in the aforementioned embodiments may beachieved by two or more components, and a function of one component maybe achieved by two or more components. Moreover, functions of two ormore components may be achieved by one component, and a functionachieved by two or more components may be achieved by one component.Some of the components of the above embodiments may be omitted. At leastpart of the configurations of the aforementioned embodiments may beadded to or replaced with other configurations of the aforementionedembodiments.

What is claimed is:
 1. An exhaust pipe comprising: a tubular portionforming an exhaust flow path in an exhaust system of an internalcombustion engine; and a space forming portion provided along at leastone of an inner surface and an outer surface of the tubular portion andforming at least one branch space between the space forming portion andthe tubular portion, the at least one branch space communicating withthe exhaust flow path and having a function as a side-branch muffler,wherein the at least one branch space comprises a space with a lengththat is one fifth or more of an entire length of the exhaust system. 2.The exhaust pipe according to claim 1, wherein the tubular portion is alongest tubular portion of a tubular body forming the exhaust flow pathin the exhaust system.
 3. The exhaust pipe according to claim 1, whereinthe at least one branch space comprises a space with a length that isone third or more of the entire length of the exhaust system.
 4. Theexhaust pipe according to claim 1, wherein the at least one branch spacecomprises two branch spaces with lengths different from each other. 5.The exhaust pipe according to claim 1, wherein the at least one branchspace comprises two branch spaces each having a length that is one fifthor more of the entire length of the exhaust system.
 6. The exhaust pipeaccording to claim 1, wherein the space forming portion is providedalong the inner surface of the tubular portion.
 7. The exhaust pipeaccording to claim 1, wherein the space forming portion comprises atubular inner pipe member disposed inside the tubular portion andforming a double-wall pipe together with the tubular portion, whereinthe inner pipe member comprises: an attached portion attached to theinner surface of the tubular portion; and a separated portion spacedapart from the inner surface of the tubular portion, and wherein the atleast one branch space comprises a space formed between the separatedportion and the tubular portion.
 8. The exhaust pipe according to claim7, wherein an area defined by an outer edge of the separated portion ina cross-section perpendicular to an axial direction of the double-wallpipe is encompassed by an area defined by an outer edge of the attachedportion in the cross-section.
 9. The exhaust pipe according to claim 1,wherein the tubular portion comprises a curved portion forming a curvein the exhaust flow path.